Which value must be added to the expression x^2 +16x to make it perfect-square trinomial?

a-8

b-32

c-64

d-256

edit: it’s not a

X^2 + 16x + ?
a^2 + 2ab + b
(a + b)^2 which is a perfect square.

To find b to determine the last value, you divide 16x by 2x.
so:
b = 8
the last value would be 8^2 which is 64.

Hence answer is C - 64